The Great Divide forms the southern stretch of Rocky Mountain border between Alberta and British Columbia.

Rain falling on the western side flows toward the Pacific. On the eastern slope, it gurgles toward the Atlantic or Arctic oceans.

But this geographically defining landmark turned politically rocky this week with two premiers under the NDP flag flapping madly off in different directions over the $7-billion Trans-Mountain pipeline expansion.

B.C. has thrown consultation foot dragging and proposed oil volume restrictions at thwarting the megaproject. A furious Alberta has over-reacted in response.

This is not a dispute about the law. That much has been decided in Alberta’s favor.

This is mostly a fight for survival by a pair of premiers whose electoral futures may well hinge on a pipeline running in their favor or being turned back.

B.C.’s John Horgan must be seen delivering on his pipeline-obstructing promise to appease his base and his Green party coalition partner.

Alberta’s Rachel Notley must have an ocean-connecting pipeline to call her own before facing voters next year.

So far, Prime Minister Justin Trudeau seems content to let the war of words and symbolic retaliations linger before his government acts. But act he must. And in 2018, not beyond.

If gimmicky tools from Horgan’s tickle box defeat a proposed pipeline which has been thoroughly prodded and probed, assessed and analyzed, approved and cabinet-confirmed according to increasingly stringent rules and regulation, well, there’s no such thing as the national interest across Canada anymore.

Comparisons between Trans Mountain and the 1885 trans-Canada railway are valid. The CPR was deemed a national economic necessity and railroaded to completion by Ottawa without considering provincial concerns.

Trans Mountain will be the only made-in-Canada outlet for oilsands bitumen to ocean tankers and overseas markets. It cannot be blocked by single-minded provincial considerations.

Notley’s retaliatory B.C. wine ban may be politically petulant but, as an attention grabber, it was ingenious. It’s a message in a bottle reminding Horgan that needlessly antagonizing a neighbor has economic consequences.

But her ban plan should not be expanded. Nor should B.C. retaliate by blocking Alberta beef imports. All that does is hurt innocent entrepreneurs and deprive consumers of choice.

Alberta should put a little water in its whine and wait a bit for the federal cavalry to arrive, waving its unilateral constitutional powers like a sword.

Ironically, that’s something Horgan also desperately needs as political cover for his inevitable failure to stop the project.

Having two of Canada’s most important provinces this greatly divided is bad for the NDP brand, bad for the economy and ultimately bad for Confederation.